1. Field of the Invention
The present invention relates to a supply voltage sensing circuit for sensing a variation in supply voltage.
2. Description of the Related Art
A supply voltage sensing circuit is a circuit that senses a supply voltage and provides a detection signal when the supply voltage rises to a certain range and when it lowers to a certain range. The supply voltage sensing circuit is used in an SRAM, a DRAM, an EEPROM, an FeRAM (Ferroelectric Random Access Memory) and the like that require fast detection of a variation in supply voltage.
One of conventionally proposed supply voltage sensing circuits includes a p-type MOS transistor, which has a source supplied with the supply voltage, a drain grounded via a current control resistor, and a gate supplied with an output voltage from a divider circuit that resistance-divides the supply voltage. In this arrangement, when the supply voltage rises above a desired value, the p-type MOS transistor turns on and senses a rise in the supply voltage. On the other hand, when the supply voltage lowers below a desired value, the p-type MOS transistor turns off and senses a drop in the supply voltage.
In the supply voltage circuit thus configured, if the supply voltage drops slower compared to the RC time constant of the current control resistor and thus the RC time constant is negligible, the drop in the supply voltage can be sensed without problems. If the supply voltage drops faster compared to the RC time constant, a problem arises because the drop therein can not be sensed. Namely, when the supply voltage drops faster, the potential difference between the gate and the source of a p-type MOS transistor lowers below the threshold voltage to turn off the transistor. Even in such the case, the RC time constant prevents a drop in drain voltage. This is not transmitted to the following stage circuit and causes a problem because a drop in the supply voltage cannot be sensed.
To solve such the problem, JP 2002-300020A (FIG. 1 and paragraphs 0032-0044) discloses a supply voltage sensing circuit as known. The circuit includes a p-type MOS transistor having a source given the supply voltage via a RC delay circuit. In the sensing circuit, when the supply voltage drops, the gate voltage lowers while the source voltage lowers with delay because of the presence of the RC delay circuit. Therefore, when the supply voltage lowers below a certain value, the p-type MOS transistor turns on. Sensing the p-type MOS transistor being turned on enables a drop in the supply voltage to be sensed. As for a rise in the supply voltage, the above conventional circuit is separately provided in parallel for detection. In the circuit of JP 2002-300020A, the CR time constant of the current control resistor connected between the drain of the p-type MOS transistor and the ground terminal cannot affect thereon and accordingly a fast drop in the supply voltage can be sensed as well.
In the circuit of JP 2002-300020A, however, the threshold voltage on the p-type MOS transistor has a temperature dependence. Due to variance factors including such the temperature dependence, a problem arises because an output timing of a power-off signal to sense the drop in the supply voltage varies.